1. Field of the Invention
The present invention relates to position control methods and systems for satellites and spacecraft, and more specifically, to controlling transition from thruster control to momentum wheel control in a three-axis stabilized spacecraft.
2. Description of Background Art
U.S. Pat. No. 5,222,023 issued to Liu et al. on Jun. 22, 1993 and entitled "Compensated Transition For Spacecraft Attitude Control" describes an improved method for transitioning a spacecraft from a thruster controlled, station keeping mode to an operational, on-orbit mode, in which attitude control is maintained using internal momentum wheels. The method first utilizes a thruster compensation technique to supplement the conventional transition mode control system. This compensation uses empirical thruster data to derate thruster efficiency for extremely short pulse durations necessarily used in transition mode operation. Secondly, a three step deadbeat thruster sequence is employed in which a first pulse linearizes spacecraft motion prior to orienting and stopping nutation. This linearization provides improved accuracy in computing and shortens the waiting time for subsequent thruster firings.
U.S. Pat. No. 4,931,942 issued to Garg et al. on Jun. 5, 1990 and entitled "Transition Control System For Spacecraft Attitude Control", describes a method for controlling nutational motion during spacecraft transition from a station-keeping mode to an on-orbit mode using a feedback control system to control multiple thruster pulse firings. Although the problems of thruster non-idealities and orbital dynamic nonlinearities were raised, no solutions were offered beyond convergence to stability through successive feedback controlled thruster pulses.
U.S. Pat. No. 4,288,051 issued Sep. 8, 1991 to Goshcel entitled, "Method and Apparatus For Controlling A Satellite", relates to the stabilizing of a satellite relative to the three major axes prior to the point in time when the satellite is to change orbits, whereupon the engine system for reaching the new orbit is switched on. No separate nutation-damping scheme is disclosed.
U.S. Pat. No. 4,537,375 issued Aug. 27, 1985 to Chan entitled, "Method and Apparatus For Thruster Transient Control" describes a method of pre-biasing individual thruster motors to compensate for motor offsets and mismatches. The damping of nutational motion is not addressed.
U.S. Pat. No. 4,725,024 issued Feb. 16, 1998 to Vorlicek entitled "Method For Spinning Up A Three-Axis Controlled Spacecraft" describes a method for spinning-up a satellite about its roll axis prior to firing a motor. Nutational motion compensation is not described.
U.S. Pat. No. 4,758,957 issued Jul. 19, 1988 to Hubert et al., entitled "Spacecraft Stabilizing System And Method" discloses a method for simultaneously processing and nutation-damping a spinning spacecraft that includes thruster firing in response to feedback from angular momentum gyros. This patent has no disclosure of the subject three-pulse thruster firing scheme, nor does it address the topic of thruster compensation.
U.S. Pat. No. 3,866,025 issued Feb. 11, 1975 to Cavanaugh entitled "Spacecraft Attitude Control System", discloses a spacecraft attitude control system for producing thruster firings which align the total angular momentum vector with the desired orbit normal vector while simultaneously adjusting orbit. The invention employs a commonly-available sensor and thruster system. However, it seeks to perform an orbit adjustment maneuver in a manner that is intended to minimize build-up of nutation during the maneuver, rather than provide a mechanism for correction following the maneuver. The control system computes angular rates rather than directly sensing the rate. In addition, the patent is silent about the specific transition attitude control techniques of the present invention.
Other patents uncovered which contain additional information on the general topics of nutation, attenuation, correction in spacecraft systems and the like are as follows:
______________________________________ U.S. Pat. No. Inventor ______________________________________ 3,624,367 Hamilton, et al. 3,643,897 Johnson, Jr. 3,937,423 Johansen 3,944,172 Becker 3,984,071 Fleming 3,997,137 Phillips 4,023,752 Pistiner, et al. 4,174,819 Bruederle, et al. 4,370,716 Armieux 4,386,750 Hoffman 4,521,855 Lehner, et al. ______________________________________